The present invention relates to the manufacture of pure metal halides which can be used, for example, in the manufacture of pure metal halide glasses or the like. More particularly, the invention relates to the manufacture of metal halides by vapor phase deposition processes wherein metal halide deposits of enhanced purity may be obtained.
Metal halide compositions are presently of interest in connection with the development of new, transparent glassy materials exhibiting unique optical properties. For example, in the field of glass optical waveguides, which are transparent glass filaments used to transmit light signals for communication purposes, glasses of improved transparency are continually being sought.
Commercial glass optical waveguides are presently formed of oxide glass materials, typically fused silica and doped fused silica glasses. Improvements in such materials have continued to the point where loss coefficients near the theoretical minimum of 0.1 db/km at 1.6 microns have been achieved. However, it has been recognized that even lower attenuations, perhaps as low as 0.001 db/km, might be attainable in halide glass systems which can operate at wavelengths further into the infrared region. Among the halide glasses which have been considered for the manufacture of extremely low loss optical waveguide fibers are glasses based on BeF.sub.2, ZrF.sub.4 and ZnCl.sub.2.
Examples of patents disclosing the use of metal halide glasses for infrared transmitting optical devices are U.S. Pat. Nos. 4,189,208, 4,308,066, and 4,343,638. These patents, however, teach preform or filament manufacture by conventional batch melting and forming methods.
Vapor deposition methods for making metal halide compositions are disclosed in U.S. Pat. No. 4,378,987 and in published Japanese Patent Application No. 57-051146. These publications recognize that vapor deposition methods may be needed to achieve the purity necessary for very low attenuation optical products.
In the method of U.S. Pat. No. 4,387,987, a particulate metal halide precursor such as a metal fluoride powder is generated and deposited on a preform substrate by reacting a vaporous metal source such as an organometallic compound (e.g., a metal alkyl or a metal beta-diketonate) with a vaporous halogen source such as HF. The reactive vapors are flowed into a reaction zone adjacent the substrate where they react to form the precursor, and this precursor is directly deposited on the substrate where it can be further processed by consolidation to form a transparent optical preform or fiber.
Another approach to the formation of pure metal halide precursor materials involves the controlled decomposition of a halogenated metal beta-diketonate to form a metal halide in the absence of added halogenating agents. In this procedure, the halogenated metal beta-diketonates are transported into a reaction zone where they are decomposed by heat or plasma energy, the resulting metal halide product being formed by intramolecular fluorine transfer and thereafter deposited on an adjacent substrate. Methods for carrying out these procedures are described and claimed in the copending, commonly assigned application of J. M. Power et al., Ser. No. 544,129, filed October 21, 1983, now U.S. Pat. No. 4,718,929.
In these and other vapor phase methods for making pure metal halide compositions involving the use of organometallic starting materials, contamination of the metal halide deposit by pyrolytic carbon from organic substituents in the starting materials can occur if the reaction is not fully controlled. Avoidance or minimization of the carbon contamination may be desirable where some important property of the metal halide, such as its optical transparency, is undesirably affected thereby.
It is therefore a principal object of the present invention to provide a procedure whereby unwanted carbon contamination in metal halide deposits produced by vapor phase deposition processes may be minimized or prevented.
Other objects and advantages of the invention will become apparent from the following description thereof.